1. Field of the Invention
The invention relates generally to an electromagnetic bioimpedance apparatus and, more particularly, to noninvasive imaging using nearfield electromagnetic holography, and especially for imaging human prostate tumors.
2. Description of the Related Art
Prostate cancer is the most commonly diagnosed cancer and the second leading cause of cancer death in American men. In a diseased human prostate, localized tumors can be pea-sized with distinct boundaries, located at the surface or at a depth, though most tumors are within 1 cm of the surface. Extensive screening, consisting of digital rectal examinations (DREs) and prostate-specific antigen (PSA) level tests, has resulted in a rise of diagnosed cases of prostate cancer. In 1996, there were more than 317,000 new cases of prostate cancer and more than 41,000 prostate cancer deaths. Currently, in men with an abnormal DRE and/or elevated PSA and who are candidates for therapy, a transrectal ultrasound with prostatic biopsy is performed.
Of men who have abnormal DRE's, one in four men will have no identifiable pathology on biopsy. Also, using an ultrasound-guided biopsy, there is a 12.8% false negative rate, meaning prostate cancer will be missed, especially for tumors in depths and surfaces that the DRE cannot reach. There has not been good correlation between ultrasound findings of hypoechoic areas and cancer. Therefore, a more sensitive and specific imaging modality to better direct prostate biopsy is needed to aid the clinician in detecting and localizing cancer.
Bioimpedance is an electrical property of biological tissue that has been used for several biomedical applications, such as quantification of brain edema in neurosurgery and in differentiating a pulmonary mass such as cancer from pneumonia. The electrical bioimpedance of a body segment depends upon a number of factors, including hemoperfusion, that is, the volume of blood or fluid in the body segment, and the electrical conductivity of the body segment.
Other biological variables that could affect electrical bioimpedance of a body segment include differences in body size, body shape, electrolytes, fluid distribution, or other elements of body composition. In addition, variations in electrode position and machine settings, hydration status and ambient air and skin temperature can each play a role in electrical bioimpedance of a body segment. Recent physical activity can increase vascular perfusion and warming of muscle tissue, increasing cutaneous blood flow and vasodilatation that affects the electrical bioimpedance.
Direct bioimpedance measuring systems utilize a current generator to generate a continuous, constant amplitude and frequency current though a human or animal body segment, such as muscle, fat, liver, skin or blood. Frequencies in the range of about 30 KHz-30 MHz have been used to determine tissue conductance (or, reciprocally, tissue impedance) in order to assess the fluid content of brain matter. Impedance to the continuous current flow in the body segment generates a voltage difference across the body segment. A bioimpedance meter measures the impedance in the body segment.
U.S. Pat. No. 4,805,621 to Heinze et al. discloses an apparatus for measuring the impedance of body tissue. The apparatus has a signal source connected to the tissue to be measured, a unit for acquiring an impedance signal from the body tissue dependent on the electrical signal, and an evaluation stage for the impedance signal. The signal source supplies an electrical signal to the body tissue to be evaluated. The evaluation stage filters out low frequency signal components corresponding to the conductance of the tissue, and has a signal output to which the signal components that were filtered out are supplied.
U.S. Pat. No. 5,529,072 to Sramek discloses a system and method for detection of electrical bioimpedance signals in a human or animal body segment. The system comprises a constant current generator for generating a periodic high frequency current output across a body segment in response to a periodic control input signal, a controller for generating a periodic control output signal to control operation of the current generator and an electrical bioimpedance detector for detecting a voltage generated across the body segment by the flow of current in the segment. The electrical bioimpedance detector generates an output signal indicative of bioimpedance in the body segment. The periodic generation of a current across a body segment alleviates the potentially detrimental effects of a continuous current on body segment tissue and reduces interference with the function of certain pacemakers.
Despite the ability of noninvasive electromagnetic bioimpedance methods to measure low contrast changes in tissue, a perennial problem has been the precise localization of the change. Generally, the interrogation or sampling volume is gauged by the diameter of the coil used to impart the electrical signal to the tissue. Because of the intervening tissue between the coil and the region under study (e.g., skin, muscle, bone), it is difficult to precisely locate the depth of the physiological change since the conductance (or impedance) of the intervening tissue is also measured. Traditionally, the signal from a receiving coil is a broadly peaked function with poor spatial resolution and no definitive electrical conductivity (or impedance) data specific to the different kinds of tissue under illumination. Additionally, bioimpedance measurements have never been used to detect prostate tumors.